1. Field of the Invention
The present invention relates to the measurement of optical properties of sea water and more particularly to improved apparatus for remotely measuring the diffuse attenuation coefficient of light in sea water from an airborne platform such as an aircraft or alternatively from a surface vessel or a submerged platform.
2. Description of the Prior Art
The optical properties of water have been of scientific interest for centuries. A number of recent theoretical and experimental studies have been conducted to investigate and better characterize the optical diffuse attenuation coefficient, K, of both fresh and sea water. The diffuse attenuation coefficient of light in water is significant in a variety of problems associated with the optical transmission qualities of sea water. For example, the performance of aircraft-to-submarine laser communications is critically dependent upon the optical transmission properties of ocean water. Other electro-optical systems used in sub-surface ocean applications are: laser radar for bathymetry hard target detection and surveillance, lidar for temperature and salinity measurements and various optical imaging systems. Each application has a strong performance dependence on the optical clarity/attenuation of sea water.
The optical clarity of water or more precisely the transmission of optical energy in water is generally characterized by an exponential decay as a function of depth, i.e., EQU I/I.sub.o =exp[-Kd] (1)
where
I is the intensity of light after traveling through water to a depth d;
I.sub.o is the initial light intensity just beneath or at the water surface; PA1 K is the diffuse attenuation coefficient at a single wavelength, and PA1 d is water depth at the point of measurement.
Expression (1) characterizes the diffuse attenuation coefficient of water only at a single wavelength .lambda., since K=f(.lambda.). The value of K at a particular wavelength also varies widely with the geographical location of the testing area. For example, clear tropical ocean waters may have transmission windows with values of K as low as 0.02 m.sup.&lt;1 at blue wavelengths i.e., (400-460 nm). Whereas, cooler ocean waters and coastal waters in general have transmission windows with higher values of K that may be as great as 0.2 m or larger throughout the blue-green part of the spectrum i.e. (400-560 nm). Ocean currents, weather and seasonal variations also affect the value of K even at a given location in the ocean. Thus, the value of K is highly variable with time and with ocean depth.
The current data base of ocean K measurements is not adequate for producing accurate statistical results to develop arithmetic means, standard deviations and cumulative probability distributions. It is apparent that the existing data base of ocean K measurements must be greatly expanded if better electro-optical system modeling and performance estimates are to be made.
Existing apparatus and techniques for measuring K have severe limitations. Hide area coverage is obtained by satellite measurements that utilize the coastal zone color scanner (CZCS). This remote technique analyzes the color distribution of reflected and upwelling ambient sunlight from ocean water (and from atmospheric radiance) and infers values of K by means of an analysis of the observed color distribution. This technique is however limited to relatively shallow depths of the order of (1/K) that is, to a depth of 20 m if K=0.05 m.sup.-1, and because of the remoteness of the satellite, its lateral spatial resolution is necessarily limited to approximately 800 m.
More direct measurement systems use photometers that are physically placed in the water and compare the intensity of sunlight as a function of depth with that obtained at the surface with a similar photometer. Such direct methods are inherently slow in generating data, require considerable manpower and are prone to inaccuracies resulting from changes in environmental conditions at the measuring location. Further such manual techniques are not suitable for rapidly and repeatedly gathering data over wide areas of the ocean and thus have limited utility.
The most venerable available device for directly measuring K is known as the Secchi disc and is described in "The Oceans" (Prentice-Hall, Englewood Cliffs, N.J.) 1982, at page 82. Basically, the Secchi disc is a round white disc 30 cm in diameter. The visibility of the disc, as seen by a human observer as it is lowered into a sunlit sea, is the measurement parameter. The depth at which the disc becomes indistinguishable is related to the value of K by the formula K=1.7/d, where d is the maximum depth of visibility in meters. This relationship varies from place to place from season to season and is highly empirical. Like the satellite measurements, this technique is limited to relatively shallow depths in the order of (1/K). Also, such techniques are manpower limited suggesting that an extensive world-wide survey would be too expensive to be practical. Moreover, relating this inferred K to a K at a specific wavelength is not a settled scientific issue.
This invention is directed toward improved apparatus for remotely measuring the diffuse attenuation coefficient which overcomes the above disadvantages.